/Engine/Source/Runtime/AnimationCore/Public/Constraint.h
Apply Constraint : Apply Constraint transform to BlendHelperInLocalSpace in local space
BlendHelperInLocalSpace will contains constraint's local transform result, it is local because that's how you want to compose multiple to one transform at the end
virtual void AccumulateConstraintTransform ( const FTransform & TargetTransform, const FTransform & CurrentTransform, const FTransform & CurrentParentTransform, float Weight, FMultiTransformBlendHelper & BlendHelperInLocalSpace ) const